FR2753457A1 - Passivation of surface defects on diamond - Google Patents

Abstract

A method for treating a diamond surface containing stressed surface defects comprises deposition a carbide forming metal, e.g. titanium and then removing it to partially passivate the surface defects, e.g. grain boundaries, twin defects or damage produced by polishing.

Description

PROCESS FOR TREATING A DIAMOND SURFACE

  This invention relates to diamond processing.

  Diamond is an extremely hard and wear resistant material which can be used in a wide range of applications. One of these applications relates to an exterior window for missiles.

  them and similar devices. In such applications, it is important

  as long as the window effectively transmits infrared and similar radiation with good efficiency. It has been found that grain boundaries, twins and polishing damage behave like regions in which the diamond can degrade when it

  is exposed to air at high temperature. Such damage reduces the ef-

  diamond efficiency and performance with respect to the transmission of infrared and other electromagnetic radiation. Faults

  which are attacked diffuse and absorb the radiation.

  In accordance with the present invention, a method of treating

  ment of a diamond surface includes the steps of de-

  lay on the surface a layer of a metal forming a carbide, and

  then lift the layer. It has been found that such a treatment of the surface of the diamond has the effect of passivating surface defects subjected to stresses in the diamond, such as grain boundaries, twins and

polishing damage.

  The diamond can be natural or synthetic diamond, but it

  will generally be diamond produced by chemical phase deposition

  fear (or CVD). CVD type diamond is diamond made by any of many chemical vapor deposition processes

  which are known in the art. These processes generally include

  The operations which consist of forming a mixture of hydrogen or oxygen and of a suitable carbon gas compound, such as a hydrocarbon, in applying sufficient energy to this gas to dissociate the hydrogen or oxygen into hydrogen or atomic oxygen, and the gas as active species consisting of ions or carbon atoms, or CH-type radicals, and allowing these species to settle on a substrate to form diamond. Gas dissociation can be achieved.

  using a hot filament, microwave or radio frequency energy

  quence, or direct current discharges.

  In one form of the invention, the surface of the diamond which is

  treated is a surface that has been polished. We have found that polishing has tended to

  dance to introduce surface defects in the diamond. These can-

  can be passive by the method of the invention.

  The metal forming a carbide may be in the form of a metal itself or be part of an alloy. Examples of suitable carbide-forming metals include titanium, molybdenum, tungsten, tantalum, niobium, hafnium, scandium, vanadium, chromium,

  zirconium, lanthanum, rhenium and silicon.

  The carbide-forming metal can be deposited on the surface of the diamond by any method known in the art, such as vacuum sputtering, vacuum deposition, chemical vapor deposition or the like. The conditions which are used in such deposition methods inevitably cause the carbide-forming metal to form a certain carbide with the surface of the diamond on which it is deposited. Carbide formation can be promoted by heat treatment of the deposited metal layer, typically at a temperature in the range of 200 C to 1300 C, in an atmosphere which prevents degradation of the diamond. Such atmospheres are by

  example vacuum or an inert or non-oxidizing atmosphere.

  It is believed that a passive carbide layer is created on the surface defects subjected to stresses in the diamond, which has the effect of passivating these defects and significantly reducing their reactivity towards oxygen at temperature. high, i.e.

  temperatures up to 900 C.

  In addition, the carbide which is present on the surface defects is capable of absorbing radar frequencies. This is important for many domes and infrared (IR) windows which must provide some degree of electromagnetic shielding to prevent radar

  does not "interfere" with the electronic circuits of a seeker. This is particularly

  this is particularly the case when the metal forming a carbide is a metal which forms

  electrically conductive carbides, such as titanium carbide.

  This leads to circular grain boundaries which are conductive and absorb radar frequencies, so that diamond having a surface thus treated is useful as an anti-radar screen. The carbide which remains on the surface defects, after the removal of the layer, can

  be chosen so as to exclude wavelengths of se-

  and behave like a filter.

  It is also possible to convert the carbide which remains on the surface defects, after the removal of the layer, to another form, for example an oxide or a nitride, to increase the transmission.

  radiation such as IR radiation.

  The removal of the metal layer forming a carbide which is deposited will typically be accomplished by dissolving in a

  acid. In general, a strong inorganic acid, such as a me-

  mixture of hydrofluoric and nitric acids, although it is also possible

  to use mechanical means such as sanding.

  The surface treatment of the diamond in accordance with the procedure

  The effect of the invention has the effect of strengthening the resistance to oxidation of the surface of the diamond at high temperature. This improves the efficiency and yield of the diamond for the transmission of IR radiation and similar radiation. The method of the invention is therefore especially applicable to the treatment of at least one of the surfaces of a window or a diamond dome, in particular a window or a diamond dome

CVD type.

  The invention will now be illustrated by the following examples.

EXAMPLE 1

  It was cleaned and placed in a spray chamber.

  a CVD type diamond film having undergone a double po-

  smoothing. Half of the bare face was covered by a mask with holes.

  There was a vacuum in the system up to 2.66 Pa (before increasing the

  pressure with pure argon), and a pre-spraying was carried out

  method of the target in pure argon for five minutes. We have

  afterwards pre-attack the sample using energy

  radiofrequency (RF) (with a polarization of -800 V) for two half

  nutes, to eliminate any surface contamination. After two minutes,

  we increased the power applied to the target and we decreased the voltage

  bias polarization of the substrate up to 60 V, to give a clear deposit of titanium on the unmasked region of the diamond film of the type

  CVD. The deposit continued for ten minutes.

  We then turned the sample over and masked the op-

  asked before following the deposit procedure described above.

  The two layers of titanium formed a certain carbide with

the diamond.

  The treated film was placed in an inorganic acid bath

  strong, such as a mixture of hydrofluoric and nitric acids, to remove

see the titanium layers.

  The film thus treated was then subjected to an attack by air at about 900 ° C. for 1 minute, and the IR transmission was measured. It was found that the regions of the diamond film to which a layer of titanium had been applied exhibited unchanged or improved IR transmission possibilities, while the untreated regions exhibited reduced IR transmission possibilities,

  cause of diffusion and absorption at the level of attacked defects.

EXAMPLE 2

  In this example, we compare a sample of untreated CVD type diamond with a sample of treated CVD type diamond

by the method of the invention.

  (A) Untreated sample Double polishing was applied to a diamond sample

  CVD type 300 Flm thick, to obtain a fine, transparent finish

  optically related, using milking techniques

  classic. We carefully cleaned the sample and we photo-

  graphed its surfaces using the Nomarski technique.

  A small Carbolite-type oven was used to heat the sample in an air environment at normal atmospheric pressure. The untreated sample was first heated to 700 C for 1 minute, after which it was removed from the oven and photographed

  its surfaces before measuring the IR transmission. We repeated this pro-

  cedure after additional heating for 1 minute at 800 C,

900 C and 1000 C.

  Effect on surface finish Figure 1A shows the growth surface of the sample

  polished, before air attack. The surface relief is reinforced by the dis-

  positive for phase contrast and appears normal for diamond type

  High quality CVD. Figure lB is a photograph with a large-

  higher smoothness and it shows crystallographic defects and grain boundaries. Figures 2A and 2B show the surface of the substrate

  polished, before air attack. The surface is smooth and devoid of

  tails, without visible grain boundaries and with very few bites.

  Figure 3 shows the growth surface after it has been subjected to air attack at 700 C for 1 minute. There is no sign of any deterioration in the surface finish. However, as shown in Figure 4, the surface is degraded after an attack

  by additional air at 800 C for 1 minute. Over-

  face have been formed by attack and become visible and, in particular,

  the grain boundaries have started to open.

  Although the growth surface remains intact, an air attack at 700 C for 1 minute reveals a large number of the joints

  grains in the surface of the substrate, as shown in Figure 5.

  An additional air attack at 800 C for 1'r minute causes

  severe degradation of the surface polish, as shown in the figure

gure 6.

  When the sample is returned to the oven for attack at 900 C for 1 minute, it is evident that damage located below the surface appears in the growth surface. The lines

  diagonals which are shown in Figure 7A are almost certain-

  damage caused below the surface which is occasion-

born through the polishing process.

  Air attack reveals this by increasing the size of the defect.

  It appears that a large polishing particle was "dragged" over the

  surface, inducing conchoidal fractures below the sur-

  face. This damage to the surface, invisible on the surface that has just been polished, could severely weaken the mechanical resistance of the material and reduce its resistance to attack by rain. FIG. 7 clearly shows that the surface of the untreated CVD type diamond is severely attacked by air at 900 C. FIG. 8 shows the surface of the substrate, after the attack by air at 900 C. grains have opened up even more and an attack is manifested in the

  twins and other crystallographic defects.

  An attack by air at 1000 C for 1 minute has a

  Dramatic on the surface finish, as shown in Figure 9.

  The growth surface is now covered with grain boundaries at-

  jogged and attacked crystal defects. It is interesting to note that there are certain crystallites which do not exhibit attacked defects, as shown in FIG. 9A, while FIG. 9B shows

  crystallites that have polishing damage below the surface.

  Figures 10OA and 10OB show the highly defective nature

  killer of the surface of the CVD diamond substrate which has multiple twins and other macroscopic crystallographic defects, in addition to the grain boundaries. All regions of high concentration of constraints, such as twin planes and grain boundaries, have been

  preferentially attacked by hot air.

  The air attack reveals in CVD diamond blades imperfections that are both crystallographic and treatment-induced. The substrate surface is attacked at 700 C, although the growth surface is not attacked. This is likely due to the more defective nature of the substrate surface, and the fact that it has a higher level of intrinsic stress than that of the surface

growth.

  Effect on the IR transmission properties A Perkin-Elmer FTIR device was used to carry out the IR measurements. Due to the small sample size, the

  measurements through a 2 mm opening.

  Figure 11 shows IR transmission through the sample before attack experiments, and Figure 12 shows transmission

after the air attack.

  The hot air attacks the grain boundaries which open and provoke

  f quent a diffusion, and the diffusion then decreases the RF transmission and would cause a distortion of any image observed through the sample. In the visible, the samples seem to become frosted, the

  diffusion occurring on both surfaces. It does not appear to dominate

  any mages in the mass of the material.

  The following table provides a summary of the res-

  aunt, after the air attack experiments.

  Length Not attacked Attack during Attack Wave attack 1 min at additional 700 C additional (pir) for I min for 1 min

at 800 C to 900 C

8 62% 61% 55% 42%

63% 62.5% 57.5% 44%

14 65.5% 65.5% 62% 47.5%

  These results show the catastrophic effect of hot air attack on the IR properties of the CVD diamond layer. It is therefore necessary to provide some form of protection of CVD type diamond optical elements, if they are to be used in high speed flight applications, even if the flight time

is short.

  The protection could be in the form of a coating.

  resistant to oxidation, but such coatings would degrade window transmission and would almost certainly be removed by

  erosion during the flight through the rain.

  (B) CVD type diamond with modified surface - Thermal stability

  As shown above, the hot air attacks by

  the grain boundaries and the defects, leaving the crystalli-

  your most perfect. An objective, which the process of the invention achieves, con-

  is to modify these defects to make them inert vis-à-vis the chemical attack by hot air, without significantly reducing the properties of

  overall IR transmission of the window.

  Surface modification procedure

  It was cleaned and placed in a spray chamber.

  ç thodique a CVD type diamond sample which has been subjected to a double polishing. We covered with a mask with holes half of the

  face naked. We made a vacuum in the system up to 2.66 Pa and we did

  performed a cathode pre-sputtering of the target in pure argon for 5 minutes. We then did a pre-attack of the surface

  of the sample using RF energy (with continuous polarization

  bare -800 V), for two minutes, to eliminate any surface contamination. After two minutes, the power of the target was increased and the bias voltage of the substrate was decreased (up to 60 V) to obtain a clear deposition of Ti on the unmasked region of the sample.

  CVD type diamond. The deposit continued for 10 minutes.

  We then turned the sample over and masked the op-

  asked, before following the deposit procedure described above. After the coating was applied, the sample was removed and a

  thin coating of gold on the titanium layers.

  Titanium formed titanium carbide on both surfaces on

which it was deposited.

  The surfaces of the sample were then photographed before removing the Ti / Au layers by dissolving in a mixture of acids.

hydrofluoric / nitric.

  Effect on Modified Surface Finish Figure 13A shows the growth surface of the polished and coated CVD diamond sample. The area on the left side of the

  photograph is the Ti / Au layer. Surface details are clear-

  visible in the coated region, while the uncoated region is relatively devoid of detail. Figure 13B is a photograph of

  the uncoated region, with higher magnification.

  FIG. 14A shows the surface of the substrate which also has

  a region covered by the Ti / Au layer. As expected, the surface details are considerably smaller on this surface than on the growing surface. Figure 14B shows a photograph

  of the surface of the uncoated substrate, with higher magnification.

  The sample was heated to 800 C in air at pressure

  atmospheric, inside an oven, for a period of 1 minute.

  No visible degradation of one surface or the other appeared, so that the sample was returned to the oven for one minute

  additional. Again, the surfaces were not attacked and

  no change in IR transmission was observed. This result impli-

  that the quality of the CVD type diamond has an effect on the speed of

  tack. The sample was then heated to 900 ° C for an additional minute, and this time there was noted an attack in the regions not

  treated from the growth surface, as shown in Figure 15.

  Figure 126 shows the treated area of the growth surface, and it

  should be compared with Figure 15.

  The treated area was not attacked in the same way as

  the treated area, and in particular the grain boundaries have not been revealed

  injured by the attack. The surface of the treated area appears to be attacked evenly, and a slight "mottling" effect can be seen. On the contrary,

  the untreated region of the growth surface has all the forms

  my characteristics that we see in figure 7.

  Effect on IR transmission properties Figure 17 shows IR transmission through the sample

  treated before attack by hot air (line "before"). The fringes of interfe-

  frequencies seen at high wave numbers are probably oc-

  caseed by a thin coating on the diamond surfaces. The fi-

  gure 17 shows the IR transmission after the attack at 900 C. It appears

  that the treated area has increased transmission, while the transmission-

  Untreated areas decreased as expected.

  Additional heating in air for 1 minute at 1000 C led to sample attack in the normal way in all regions of the two surfaces, giving an identical appearance

to that of FIGS. 9 and 10.

  Surface analysis of modified CVD diamond samples A secondary ion mass spectroscopy (or SIMS) study was performed to characterize the treated and untreated diamond surfaces. In this example, the surface was modified with Ti. We have

  found that the untreated region was devoid of Ti, while the re-

  treated region showing the Ti which had reacted with the diamond (forming a TiC compound) and which had remained after the consumption of the Ti layer. SIMS analysis also showed that the modified surface contained both oxygen and nitrogen, while these bodies were absent from the untreated region. These are low levels of impurity resulting from the deposition of Ti and the reaction between the thin layer of TiC and the air. Conclusion Experiments have shown that crystallographic defects (including grain boundaries) present in CVD type diamond are attacked by air at high temperature. Attack speed depends

  temperature and reaction time, although noticeable effects

  were observed at 700 C for an attack time of only one minute. The surface of the substrate, which has more defects and which is subjected to greater stresses than the growth surface,

  is attacked at a temperature lower than that of the growing surface

sance.

  After a reaction time of only one minute at 800 ° C, polished surfaces attacked by hot air scatter IR light to an extent that makes the window virtually useless for applications

fast missiles.

  Windows which have undergone a surface modification (by pul-

  cathodic verification of titanium, with polarization, and acid removal of the thin coating of titanium on the surfaces of the diamond) provide protection to grain boundaries (and other crystallographic defects)

  against erosion by hot air. Processed samples do not show

  No reduction in the IR transmission properties is attempted, even after attack by hot air for 1 minute at 900 C. It is believed that Ti forms a layer of passive carbide at the grain boundary surfaces,

  and that this layer significantly reduces the reaction with hot air.

\ 112753457

Claims (11)

  1. A method of treating a diamond surface containing surface defects subjected to stresses, characterized in that it   includes the steps of depositing a layer of metal on the surface   mant a carbide, and then removing the layer, which has for   passivating effect of surface defects subjected to stresses.   2. Method according to claim 1, characterized in that the   surface defects are grain boundaries, twins or damage polishing ages.   3. Method according to claim I or claim 2, ca-   characterized in that the metal forming a carbide is selected from titanium, molybdenum, tungsten, tantalum, niobium, hafnium, scandium, vanadium, chromium, zirconium, lanthanum, rhenium and silicon.   4. Method according to any one of the preceding claims.   teeth, characterized in that the metal forming a carbide is deposited on the surface of the diamond by a process selected from sputtering under vacuum, vacuum deposition and chemical vapor deposition.   5. Method according to any one of the preceding claims.   teeth, characterized in that the metal layer forming a carbide which   is deposited is removed by dissolution with acid.   6. Method according to claim 5, characterized in that   the acid is a strong inorganic acid.   7. Method according to claim 6, characterized in that   the acid is a mixture of hydrofluoric and nitric acids.   8. Method according to any one of the preceding claims.   teeth 1 to 4, characterized in that the metal layer forming a car-   bure that is deposited is removed mechanically. - <2- 2753457 - and2   9. Method according to any one of the preceding claims.   teeth, characterized in that the diamond is CVD type diamond.   10. Method according to any one of the preceding claims.   teeth, characterized in that the diamond surface is a po- lie.   11. Method according to any one of the preceding claims.   teeth, characterized in that the surface of the treated diamond is an over- in front of a diamond window.